Validate your XHTML and XML documents with XML Nanny
Here is a free Mac OS X utility by Todd Ditchendorf for all you concerned web designers out there. XML Nanny allows you to validate your XML and XHTML documents (either locally or over a network) via a nice Aqua interface. If you're working on a compliant site that is behind a firewall, the W3C web-based validation service may not be able to get at it. This is were XML Nanny is helpful. Did I mention it's free? Check it out if standards-compliant web projects are your bag.
[Via What Do I Know?]